It's in drop #A but tuning doesnt really matter because the problem is still there, no matter what tuning. There's nothing wrong with my strings either, as I have tried to change them, but the problem still remains.

Is the string pretty dirty? If so it might just need changed. If not, then it could well be the action or intonation. Try getting a full setup sometime by a professional. It might just be slightly hitting a fret where it shouldn't lower down the neck. Or it might not be sitting in the saddle correctly.